Incipient sexual isolation in the nasuta-albomicans complex of Drosophila:mating preference in male-, female- and multiple-choice mating experiments

Interracial divergence is an important facet of speciation. The nasuta-albo micans complex of Drosophila with sixteen morphologically identical, karyot ypically different but cross-fertile races is an excellent system to study a few dimensions of raciation. Drosophila nasuta nasuta, Drosophila nasuta albomicans, Cytorace 1, Cytorace 2, Cytorace 3 and Cytorace 4 of this subgr oup have been subjected to male-, female- and multiple-choice mating experi ments. Out of 8456 crosses conducted, 7185 had successful matings. The over all impression is that mating is far from random amongst these six closely related races of the nasuta-albomicans complex. The males of D. n. albomica ns, Cytorace 1 and Cytorace 4 in male-choice, the females of Cytorace 1 and Cytorace 2 in female-choice, and the males and females of D. it. nastua, D . it. albomicans, Cytorace 1 and Cytorace 4 against the males and females o f Cytorace 2 in multiple-choice experiments, had significantly more homogam ic matings than expected. Thus in this study of evolutionary experimentatio n on raciation under laboratory conditions, we have documented the initiati on of preference for con-specific matings among closely related and indepen dently evolving members of the nasuta-albomicans complex of Drosophila.